1. Field of the Invention
The invention pertains to the general field of optical multiplexers and de-multiplexers and, in particular, to a device for improving the bandwidth and spectral uniformity of multiple-channel optical systems.
2. Description of the Prior Art
In dense wavelength division multiplexing (DWDM) optical communication, various frequencies (1/λ) of laser light are used as carrier signals (channels) and are coupled into the same optical fiber, which acts as a waveguide. Data signals are superimposed over the carrier signals and are transported in the waveguide. Thus, the information capacity is directly proportional to the number of channels in the fiber. Since the total usable wavelength range is limited (about a few tens of nanometers), as channel spacing decreases, more channels can fit into the same optical fiber and greater communication capacity is achieved.
Channel spacing is limited by the capability of the multiplexer (MUX) and the de-multiplexer (de-MUX). Currently, the standard channel spacing is 100 GHz (0.8 nm) and manufacturing costs increase dramatically to implement a channel spacing smaller than 100 GHz. Various methods are known in the art to multiplex and de-multiplex signals with different carrier frequencies (wavelengths). When the total number of channels is less than about 20, the technology based on thin-film filtering is preferred because of its wide bandwidth, its good thermal stability, and the facility with which channels may be added to the system. However, since the channels are de-multiplexed by cascading filters in series, the insertion losses are not uniform among the various channels. In addition, when the channel spacing is about 50 GHz or smaller, narrow-band filters based on thin-film technology add too much chromatic dispersion for some applications.
Therefore, when the number of channels is high (e.g., more than about 40), it has been preferable in the art to use optical devices that provide a more uniform loss throughout the channels and exhibit a smaller chromatic dispersion than thin-film technology. For example, devices based on array waveguide grating (AWG) and diffraction grating provide these advantages. However, such devices tend to produce a narrower bandwidth than thin-film technology, which severely limits their application. Therefore, a cost-effective method for increasing the bandwidth of multiplexing and de-multiplexing devices with uniform insertion loss throughout the channels and minimal chromatic dispersion would be very desirable.
For example, FIG. 1 illustrates the typical spectrum of a grating de-multiplexer. The insertion loss of the system is shown as a function of normalized frequency. As normally done in the art, the normalization factor is the free-spectral range of the spectrum (e.g., in a 100 GHz DWDM system, the normalization factor is 100 GHz). The spectrum 10 of each channel is characteristically dome shaped at the center frequency of each channel, which produces a very narrow bandwidth at half dB (about 0.23 units of normalized frequency). In a 100 GHz system, this corresponds to 23 GHz, which is too narrow for most applications. As one skilled in the art would readily understand, the so called bandwidth at “half dB” corresponds to the bandwidth within which the insertion loss of a device is less than approximately 10% from peak and is a measure of the insertion loss variation normally tolerated within a bandwidth in the art. Therefore, the general objective of the invention is to suppress transmission near the center frequency of each channel to produce a more uniform (flat) spectral response, such that as much as possible of the insertion-loss spectrum in each channel falls within the half dB range.
This objective could be achieved in straightforward manner by inserting a filter in each channel with an insertion loss that has maximum magnitude in the neighborhood of the center frequency of the channel. However, multiple filters would be required with significant cost increase to the system. The invention provides a single optical device with an insertion-loss spectrum characterized by periodical maxima (in magnitude) that can be selected to coincide with the pass-band center frequencies of the grating de-multiplexer. In addition, the shape of the insertion loss can be judiciously designed for a particular purpose, thereby providing a tool for attenuating the dome-shaped portion of the spectrum to increase the bandwidth of all channels in a system with a single optical device.